In the case where an NOx storage reduction catalyst (which will be simply referred to as the NOx catalyst hereinafter) is provided in an exhaust passage of an internal combustion engine, NOx stored in the NOx catalyst may be reduced by decreasing the air-fuel ratio of the ambient atmosphere around the NOx catalyst to a target air-fuel ratio. A known way of decreasing the air-fuel ratio of the ambient atmosphere around the NOx catalyst is decreasing the air-fuel ratio of the exhaust gas discharged from the internal combustion engine.
Specific methods of decreasing the air-fuel ratio of the exhaust gas discharged from the internal combustion engine include decreasing the intake air quantity, increasing the quantity of EGR gas, performing sub fuel injection in the internal combustion engine at such a time posterior to the main fuel injection that the supplied fuel is burned, and retarding the timing of fuel injection in the internal combustion engine.
Patent document 1 discloses a technology in which when NOx stored in the NOx catalyst is to be reduced, lean operation is changed into rich operation, and a large quantity of EGR gas is supplied into the cylinder.
Patent document 2 discloses a technology in which when NOx stored in the NOx catalyst is to be reduced at a time when the engine speed and engine load are high, the quantity of fresh air is decreased by increasing the quantity of EGR gas, whereby the quantity of gas passing through the NOx catalyst is decreased.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2004-360484
[Patent Document 2] Japanese Patent Application Laid-Open No. 2004-245046